How to heat citrus in winter

Hi guys
I have a mix of citrus trees I bought last year, wintered in pots, and planted in the ground this spring. None of them like to the winter, and some of them fared worse than others, almost dying. After some research, it appears that they’ll forever do poorly here unless I can prevent them from dropping below freezing during the winter. How can I do this?
The area Im in is not bad, I think it’s technically a 9B, although we can get some cold snaps here in the mountains. There’s actually a very good citrus growing area about two hours south in a microclimate area near the coast.
This fella had some good ideas:
All my trees are planted on the south facing slope, but I got a feeling they’re not going to like the winter and get set back again. There’s no power where I have them planted. I was thinking maybe some kind of heater with A thermostat and battery solar panel voltage cut off might work. Covered with floating row covers, the idea would be to heat up the trees just at the worst times, as the Guy in the video did. Sounds kind of expensive though.
Any better ideas?

Edit: just wanted to add I’m in Japan, and experimenting with a few varieties. Some are much more cold hardy than others, although I believe the fruit quality suffers on all of them if it drops below freezing. I got a little fruit off four of them last year and I thought it was terrific: Sudachi; Kabosu; Shikwasa; Ureka Lemon. Currently the kabosu has 2 large limes almost ripe, the lemon and sudachi a few blossoms turning to fruit, and the the Kinkan kumquat has exploded with hundreds of blossoms. There’s also a couple of Mikan trees and a Yuzu who’s foliage doesn’t look too bad, although I didn’t see any flowers this year. The navel orange fared the worst, looks like you’re really got the kicked out of it during the winter and was reduced down to rootstock stub. I didn’t get organized on the fertilizer until late this year, which will affect the growth. But Im mainly concerned about the coming cold this winter.
Additionally, I was surprised to hear lemons produce throughout the whole year. That would be nice. I believe the other citrus I have only produce at a specific time during the year in season.
Any ideas?

Not that I’ll be a lot of help with this, but you didn’t say how cold your lowest lows are or whether the cold is an occasional thing or lasts through the winter. I’m not at all familiar with the weather in any part of Japan. Without looking it up, I’m thinking 9B lows are 25 to 30 F, but I’m not sure. That would be tolerable for many citrus if they are hardened off when it hits, and if it’s short term. They would not be happy, and fruit would get ruined, but many trees would survive.

When protecting trees the cover should not touch them, but needs to reach the ground. Ground beneath trees should be cleared and not mulched when covered for frost so that heat can rise from the earth.

What kinds of citrus are you growing?

I was editing my first post while you were replying I guess, just noticed it now, thanks for the interest MuddyMess. I think I accidentally answered some of your questions in it. The temperatures a good question, I’m not exactly sure. January and February are cold, I think it usually stays above freezing in the day and can drop below freezing at night. We get a bit of snow, but not too much that sticks. I don’t think it ever drops below -10 Celsius.
Your comments match what I’ve seen – some people growing tougher citrus varieties around here like Yuzu and Mikan, but the fruit quality is not that great. I’m hoping to remedy that.
June-August is semi tropical day and night.

9b doesn’t sound too lethal for citrus, but may compromise fruit production and growth.
btw, when the gentleman in that video said ‘old fashioned xmas lights’ he meant the incandescent ones, as in-- thomas edison light bulbs, and not the now more common energy-efficient led xmas bulbs, as modern led bulbs generate lots of light but little heat.
what we don’t like about incandescent bulbs for lighting purposes(using more energy to generate heat than generating light) is exactly what we want when protecting our plants during winter.

one instance when ‘backward technology’ serves a purpose… Speaking of which, this backward tech might be a rarity in japan.

I’m curious to know what heating options are available. I already have a deep discharge battery, I was thinking of buying a voltage cut off regulator and solar charger. Then all I would require would be some kind of heating cables connected to a thermostat. Considering I’m just trying to stop the trees from dropping below freezing, I think it may be doable.
What do you think? Do you know of any such thermostat regulated heating cables or such?
Edit: The concept would be to have it as autonomous as possible, so the heating kicks in only when needed, and I don’t have to be there.

if you’re putting a tarp over your citrus, some space heaters commonly used in usa have thermostats which one could employ.
despite the tarp, quite possible this might not be cost-effective.

Yes, prices might get a bit silly.
This idea looks really promising:
Basically, useing water tanks as a cold frame to grow Satsuma in zone 6B. All radiant energy with no extra required to keep temps decent all winter. Amazing.
I’m just wondering about solar cooking on sunny afternoons or lack of air circulation (mold rot etc especially if fruit involved)?
I’m also wondering if there’s an easier way to accommodate my multiple small trees spread out. How about a large clear garbage bag sealed around tree ( few holes punched for air circulation ) with a bucket or jug of water sealed inside? My winters are much milder, just need to protect from occasional cold spells and arctic wind. Goal is to keep above freezing.

I think it’s Edible Landscaping that builds a frame (pvc will do) around their frost sensitive trees and bushes They enclose that with plastic sheeting and then cover that with burplap or frost blankets. Key points are that the covering should not touch the plants. It needs to come all the way to the ground and beyond so that it the warmth from the earth stays trapped and cold air doesn’t seep up from the bottom. The covers must be opened up during sunny days so you don’t overheat the plants.

Note that any freeze protection solutions aren’t just for citrus trees, but for any sensitive plants. Of course, none of the simpler ones will work if it gets too cold too long.

I was surprised though that he lives in zone 6B and was able to keep the temperature always above 27.5 Fahrenheit with no power etc., all from the water tank temp buffer.
I have a huge 500l black water tank that’s unused right now, it’d be great to try something out. Not sure if it would work to have the tank in the center and then some large covering of some kind over all the trees. Such a “covering” sounds expensive. Additionally, I’m not sure I can run up to the field every time there’s a sunny break and open up some vents and rush back when he gets cold and close it in the middle of the night or such.
As a side, my chewed up eureka lemon tree has suddenly put out about 40 blossoms. I can’t imagine it being able to support that many fruit. It would be really nice to shield the citrus somehow during the winter to allow the leaves to stay on. I think it’s a reasonable request given that I am in zone 9.

Is it’s possible to construct a cold frame of some kind that doesn’t need to be open and closed? Perhaps permanent vents that still block wind?

How about covering with a heavy frost blanket or several layers of lighter material and put some containers of water underneath. Around here with lots of sun in winter we don’t need to remove frost blanket even at 20C. The blanket should give 2-3C protection without water.

The frost blanket can rest right on the tree and needs to extend to ground burying the bottom edge.

That might work. Frost cloth is basically like a floating row cover for bugs? So maybe three layers of that tucked in around the tree, and a black bucket of water under the tree?
Or how about a huge sheet covering all the trees, and the 500l tank in the middle?
Seperate sounds more flexible and likely to work.

I’m not sure what the shape of your 500L tank is,but possibly experiment with how much water to put into it, for the Sun to heat the water,during the Winter.That’s about 100 gallons.Maybe a sweet spot can be found as to the right amount.If the tank was more flat,than barrel shaped,heating it should be quicker. Brady

Yes about three layers of the light stuff or one heavy. I’d think doing each tree separately would work best.

Here’s the tank I’ve got. It’s 132 gallons. It got too hot to touch in the summertime. But I believe the key point is the energy it takes to freeze/thaw acts as a temp buffer, as mentioned in the link above :ホームローリー-500L-雨水タンク-貯水槽・貯水タンク-スイコー/dp/B009SIKP14/ref=sr_1_1?ie=UTF8&qid=1441572996&sr=8-1&keywords=500l+タンク
Regarding frost cloth, if the heat is being provided from the ground radiantly, would there be any advantage in making a pyramid shape to capture as much ground as possible under the cloth?

If the water freezes I think your tree will already be damaged. So it’s mainly the temp change of the water.

It might help to have a frame to hold the frost blanket off the tree. But making it bigger probably isn’t worth the cost. What will help is bare soil to aid heat transfer into and out of soil. Wet soil is also an advantage. That might be an issue if frost blanket sheds water away from the tree.

Here’s a thermostatically controlled outlet you could use. I bought one and used it with Xmas tree lights (C-9 incandescent bulbs) and it worked perfectly…on at 35F(2C), off at 45F(7C). It’s called a Thermo Cube:

About your citrus and cold weather, you may be worrying excessively if my experience is any example. I’m in a 9b zone near Los Angeles. Every winter the temperature gets down to 26-28F two or three times, on the coldest nights staying below 32F from about 10PM to 6-7AM. My Bearss Lime has yet to show any bad result from this cold, and limes are supposed to be the most sensitive to cold. Sometimes I’ll throw a few layers of Agribon 19 frost cloth over it if I think about it. And I have 10 other citrus in containers (like the Bearss) and others in the ground. I’ve never seen any noticeable damage on any. My older in ground citrus survived 18F (2007) with no obvious damage. Satsumas, and some mandarins, are known for their relative hardiness and although I’ll cover some citrus if I’m expecting 24-26F nights, I still don’t worry about the Satsumas. I’m speaking of damage to the trees themselves, there could be some damage to some of the fruit, and most of my trees are still young so not that much fruit. But my in ground Golden Nugget mandarin two years ago had 40 tangerines that survived the winter(26F a few times) and ripened properly in the spring.

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I hate to bring this up, but for all the time and trouble you’re looking at, you could buy all kinds of citrus in the store for a very long time and still be ahead. In your climate I’m partial to apple trees which could care less about the cold and will still fruit well.

I’m pretty sure it gets colder for longer where I am than anywhere near L.A. Good to know the tolerance levels of various trees you have though. I really like the passive heating idea to help my trees flourish.

Applenut, we wouldn’t have much opera then, would we? lol. Actualy where I live the quality, availability, and cost of store citrus is usualy not good. I like apples, but they don’t mix well with vodka.

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